In recent years, various kinds of fuel cell assembles have been proposed as energy sources of the next generation containing a stack of fuel cells in a container.
FIG. 11 shows a stack (cell stack) of conventional solid oxide fuel cells. The cell stack has a plurality of fuel cells 1 which are arranged in alignment, a collector member 5 of a metal felt being interposed between the one fuel cell 1a and another fuel cell 1b which are neighboring each other, and a fuel electrode 7 of the one fuel cell 1a being electrically connected to an oxygen electrode (air electrode) 11 of the other fuel cell 1b. 
The fuel cell 1 (1a, 1b) comprises an electrolyte 9 and the oxygen electrode 11 of electrically conducting ceramics formed in this order on the outer peripheral surface of the fuel electrode 7 of a cylindrical cermet (the inner space is a fuel gas passage), and has an interconnector 13 provided on the surface of the fuel electrode 7 which is covered with neither the electrolyte 9 nor the oxygen electrode 11. As is obvious from FIG. 11, the interconnector 13 is electrically connected to the fuel electrode 7 but so will not to be connected to the oxygen electrode 11.
The interconnector 13 is formed by using electrically conducting ceramics that is little subject to be degenerated with the fuel gas or the oxygen-containing gas such as the air. Here, the electrically conducting ceramics must be so dense as to reliably isolate the fuel gas flowing inside the fuel electrode 7 from the oxygen-containing gas flowing on the outer side of the oxygen electrode 11
Further, the collector member 5 provided between the neighboring fuel cells 1a and 1b is electrically connected to the fuel electrode 7 of the one fuel cell 1a through the interconnector 13 and is further connected to the oxygen electrode 11 of the other fuel cell 1b. Therefore, the neighboring fuel cells are connected in series.
By containing the cell stack having the above-mentioned structure in the container, the fuel cell is used in the form of an assembly. For example, the fuel gas (hydrogen) flows inside the fuel electrode 7 and the air (oxygen) flows along the oxygen electrode 11, and electricity is generated at about 750 to about 1000° C.
In the above fuel cell, in general, the fuel electrode 7 comprises N1 and Y2O3-containing ZrO2 called stabilized zirconia (YSZ), the electrolyte 9 comprises ZrO2 (YSZ) containing Y2O3, and the oxygen electrode 11 comprises a perovskite composite oxide of the type of lanthanum manganate.
As the method of producing the above fuel cells, there has been known a so-called co-firing method which fires the fuel electrode 7 and the electrolyte 9 simultaneously. The co-firing method is a very simple process having a decreased number of production steps, and is advantageous for improving the yield of cell production and for decreasing the cost.
Here, the Y2O3-containing ZrO2 forming the electrolyte 9 has a coefficient of thermal expansion of about 10.8×10−6/° C. whereas the fuel electrode 7 supporting the electrolyte 9 contains Ni having a coefficient of thermal expansion of 16.3×10−6/t which is very larger than that of YSZ. In conducting the co-firing as described above, therefore, a difference in the thermal expansion becomes great between the electrolyte 9 and the fuel electrode 7 supporting the electrolyte arousing such problems as the occurrence of cracks in the fuel electrode 7 and exfoliation of the electrolyte 9.
As a fuel cell solving the above problems, there has been known a fuel cell obtained by forming a fuel electrode, an electrolyte and an oxygen electrode layer on a support board of a porous material which contains Ni and a rare earth oxide (Y2O3 or Yb2O3) having a coefficient of thermal expansion lower than that of ZrO2 (see patent document 1).
According to the above fuel cell, the coefficient of thermal expansion of the support board can be brought close to the coefficient of thermal expansion of the electrolyte. At the time of co-firing, therefore, it is made possible to suppress the occurrence of cracks in the fuel electrode and exfoliation of the electrolyte from the fuel electrode.    Patent document 1: JP-A-2004-146334